Agricultural tractor draft controls traditionally have been hydromechanical systems, wherein implement draft is sensed mechanically and, through appropriate linkage, a hydraulic valve is actuated to raise and lower the implement. Mechanical linkage systems have an inherent limited flexibility that has been further reduced by the implementation of environmentally isolated cabs. The development of electronic microprocessors for automotive applications has provided design engineers flexibility for improving hydromechanical systems on industrial and agricultural equipment.
In an early known system for electronically controlling the draft or position of a tractor hitch mechanism, a pump supplies a directional control valve with hydraulic oil. The directional control valve controls a hydraulic cylinder which acts to control the position of the hitch through a linkage. By this means, an attachment or implement can be raised, held or lowered. In this known system, the actual position of the links is obtained with a position sensor and the draft load with a draft sensor. In an electronic control box, the sensor inputs are compared with the set nominal value. Corrections to match the depth setting at the operating panel are triggered electronically to the directional control valve and the hitch moves until the actual value of the sensors matches the nominal value setting.
More sophisticated systems are disclosed in U.S. Pat. No. 4,132,272 issued to Gale A. Holloway et al. for a "Tractor Hitch Position Control System", and U.S. Pat. No. 4,132,273 issued to Robert M. Mortonson et al. for a "Tractor Hitch Control System Having Safety Features", both patents being dated Jan. 2, 1979 and assigned to International Harvester Company.
The Holloway et al. patent discloses a main lift cylinder controlled by a spool valve which in turn is operated by a pilot cylinder. This pilot cylinder is controlled by a control valve that preferably includes a pair of poppet valve units each operated by one of a pair of solenoids. A potentiometer may be set so as to produce an output voltage corresponding to a desired vertical position of a rockshaft and hence of the tractor hitch. The rockshaft position is sensed by a second potentiometer and a third potentiometer senses the position of the spool valve controlling the flow of fluid to the main lift cylinder. An error signal is formed by a comparator circuit, the error signal corresponding to the difference between the desired and present positions of the rockshaft. The error signal and the spool valve position are operated upon by three error detecting circuits that operate in conjunction with the pilot valve to provide dead zones of operation. That is, the error signal must be a predetermined magnitude before the pilot signal is actuated.
The Mortonson et al. patent adds a modification to the Holloway et al. system in the form of a circuit that limits the amplitude of the error signal and which in turn effectively limits the velocity of downward movement of the rockshaft. A separate manually operated switch is provided for controlling the velocity-limiting operations.
The present invention provides an electrically operated hydraulic power lift system having an electronic control system that is capable of receiving and operating on a greater number of input variables in arriving at a decision resulting in control movement of the lift mechanism, and which includes a sample-and-hold circuit in a feedback circuit to increase the expected useful life of the solenoids in the system.